Unlike other commercial oilseeds, cottonseed contains a toxic pigment, gossypol, which prevents it from being a feed for animals, other than those that have a rumen. That is, while some whole cottonseed is fed to mature ruminants, most of it is separated into oil and meal, typically by solvent extraction using hexane. As used herein, cottonseed meal, or flour, refers to the whole residue remaining after most of the oil has been removed. Before the oil and meal can be used as a food source, the gossypol must be removed or deactivated. In addition, unfavorable growing, harvesting, or storage conditions can cause cottonseed to mold and become contaminated with a mold metabolite aflatoxin, which, because it is carcinogenic, must be removed, or destroyed. The presence of such toxic components prevents cottonseed from reaching its full potential as a food source, especially in countries that grow cotton but not soybeans. In the intact cottonseed, the gossypol is concentrated in glands that are covered with a hydrophilic coating, which keeps the gossypol from coming into contact with other components of the seed. Originally, gossypol was deactivated by pressing, or expelling, the oil out of moist seeds at relatively high temperatures, such as at temperatures from about 110.degree. to 130.degree. C. Under these conditions, the glands are ruptured by hot moisture, releasing gossypol. Most of the gossypol reacts with protein, thereby forming bound gossypol, which is insoluble. The rest reacts with phospholipids and other low molecular weight components of the seed to give products that are soluble in oil and other organic solvents, as is any unreacted gossypol. The gossypol in these soluble products is referred to as free gossypol. Total gossypol is the sum of bound gossypol and free gossypol. Total gossypol content of meals made by such a process are typically from about 0.7 to 1.0 wt. %. While binding to protein is advantageous in that it acts to detoxify the gossypol, it is disadvantageous because it reduces the nutritive value of the meal by reducing the available lysine content.
Currently, separation is done by expression, or by extraction of the oil from flaked kernels at elevated temperatures with a solvent such as hexane, or a combination of expression and extraction. The most common method used today is solvent extraction, but unless a separate moist heating preconditioning step is included, meals produced by this method will contain unruptured glands and excessive amounts of free gossypol. Furthermore, it is well known that adverse physiological effects can occur with some meals containing high total gossypol, even though free gossypol is within acceptable limits.
Hence, it has long been recognized that a process is needed that could reduce the total gossypol of cottonseed meal by removing gossypol instead of binding it to protein. Various processes have been developed in an attempt to accomplish this. For example, multistep processes in which both the oil and gossypol are extracted with different solvents, in different steps, are known. For example, U.S. Pat. No. 4,359,417, teaches a two step process comprising first extracting cottonseed flakes with an 85% aqueous ethanol solution at about 110.degree. F., which removes some of the gossypol with the remainder becoming bound to the meal. This is followed by a second extraction but with a 95% ethanol solution at 175.degree. F., which removes the oil. Although such a process has merit, it can do no better than produce a meal which still contains from 0.29 to 0.45 wt. % total gossypol and 0.019 to 0.045 wt. % free gossypol.
Another reference, Canella and Sodini (Journal of Food Science, 42:1218-1219 (1977)), discloses a method wherein hexane extraction of raw cottonseed at 25.degree. C. is used to produce a cottonseed meal, followed by room temperature extraction with n-butanol containing HCl at carefully controlled pH of about 4.5, in order to obtain a product which contains 0.34 wt. % total and 0.07 wt. % free gossypol. One disadvantage of such a process is that the high boiling point of n-butanol makes its removal from the meal difficult. Further, the one-solvent processes that have been described in the art generally use mixed solvent systems which are not suitable for food use. For example, U.S. Pat. No. 3,557,168 teaches the use of a hexane-acetone mixture and U.S. Pat. No. 2,950,198 (King et al, 8/1960) teaches the use of a hexane-acetone-water mixture. Both of these processes leave residues which produce a strong objectionable catty odor in the meal, thus making them unsuitable for food use. Further, U.S. Pat. No. 4,747,979 teaches the use of a chlorinated hydrocarbon as one component of their mixed solvent, which of course is also unsuitable for food use.
Although other separation processes, such as the liquid cyclone process taught in U.S. Pat. No. 3,615,657 can yield solid products containing less than about 0.3 wt. % total gossypol from glanded seed, such low gossypol fractions account for only about 50% of the total meal. The remaining fraction contains relatively high total gossypol concentrations.
Also, U.S. Pat. No. 4,219,469, teaches the use of a solvent solution comprised of: a non-polar solvent, such as hexane; a polar solvent, such as ethanol; and a food grade acid, such as citric acid; to obtain protein isolates from cottonseed. The isolates are improved because of their improved coloration. Gossypol is not significantly removed by the process of this reference.
Furthermore, in conventional processes for producing cottonseed meal, a small amount of the gossypol which is present in the seed, is bound to phospholipids and extracted with the oil. This is in contrast to the major portion which is bound to protein and remains in the meal. Another patent, U.S. Pat. No. 3,062,876 discloses that the soluble bound gossypol can be dissolved in methyethylketone (MEK) and hydrolyzed in the presence of phosphoric acid. It is stressed that MEK be used because the bound gossypol must be soluble in the acidic solution used for hydrolysis.
The potential use of gossypol as a male contraceptive is an added incentive for the need for a process that can separate unmodified gossypol from both cottonseed oil and meal, instead of binding it to the meal. Consequently, there still exists a need in the art for more effective processes for removing gossypol, as well as other non food grade components from cottonseed.